1. Field of the Invention
The present invention relates to an exposure apparatus and a method of using an exposure apparatus for manufacturing a device, more particularly, although not exclusively, for manufacturing a semiconductor element.
2. Description of the Related Art
In a process for manufacturing a semiconductor device, such as an IC, an LSI, or a liquid crystal panel, a substrate (semiconductor wafer substrate or glass substrate) is subjected to various treatments. Among others, an exposure process for transferring a pattern is a key process in the manufacture of a semiconductor device. The exposure process is generally performed with an exposure apparatus (stepper, scanner, etc. . . . ).
A resist applied to a wafer is divided broadly into two types: a polymer film resist and a chemically amplified resist. In the polymer film resist, ionizing radiation (ultraviolet rays, X-rays, electron beam, etc. . . . ) can efficiently initiate a chemical reaction. In the chemically amplified resist, the exposure to light generates a catalyst (acid), which catalyzes image formation in a post-exposure bake (PEB). Since the image formation with the chemically amplified resist utilizes a catalyst, the sensitivity of the resist can easily be increased. Thus, the chemically amplified resist can have widely been used as a resist for an excimer laser.
However, a catalyst generated by the exposure diffuses into the air or the surface of a wafer. Thus, the post-exposure bake enhancing the catalytic action causes deterioration in the image profile. Thus, when the chemically amplified resist is used, chemical contamination due to a basic gas in the environmental atmosphere, such as amine or amide, can be prevented or reduced during the application of the resist, the exposure to light, and the post-exposure bake.
An exposure apparatus includes an illumination optical system, which irradiates a reticle (original plate) with light from a light source, and various optical elements, such as lenses and mirrors. A shorter wavelength of exposure light causes fogging of an optical element that is irradiated with the light or through which the light passes, undesirably decreasing the amount of light reaching a wafer. It is believed that the fogging is caused by an organic compound or ammonium sulfate (NH4)2SO4. Exposure to light induces a photochemical reaction of an ammonium ion (NH4)+, a sulfate ion (SO42−), compounds thereof, or an organic gas in the air, thus forming a deposit on the optical element.
As conventional countermeasures against lower solubility of the surface of a chemically amplified resist and the fogging of an optical element, an environmental chamber, which controls the temperature, the humidity, or dust around an exposure apparatus, is provided with an impurity filter to remove a substance in the atmosphere, such as a basic gas, a sulfuric acid gas, or an organic compound gas.
At the same time, components used in the environmental chamber are cleaned, for example, by washing. In addition, a lubricating agent that emits a smaller amount of such a gas component is selected. Furthermore, part of a space or the entire space around an optical path between a light source and a substrate to be treated is purged with a gas inert to exposure light.
Japanese Patent Laid-Open No. 2001-028331 discusses such a purge with an inert gas. This patent document focuses attention on a method for supplying an inert purge gas (nitrogen) into a casing of an optical device and aims to reduce contamination of an optical element, such as a lens, due to the inflow of the inert purge gas.
Specifically, the inert purge gas flowing from a gas supply unit at a predetermined flow rate is slowed down at the inlet of the casing and flows more slowly near the surface of the optical element. This allows the inert purge gas to diffuse in the casing more easily, preventing or reducing impurities in the inert purge gas from depositing on the optical element.
In Japanese Patent Laid-Open No. 2001-028331, a portion including a light source through projection optical system is hermetically sealed to isolate this portion from the outside air. This prevents or reduces impurities from depositing on the optical element in contact with the outside air.
Alternatively, a space between the light source and a wafer stage is hermetically sealed to isolate the space from the outside air. This also prevents or reduces impurities from depositing on the optical element.
However, such a hermetically sealed structure increases the size of the optical device and requires an opening and closing mechanism for sending in and taking out a mask (original plate) and a wafer (substrate). The opening and closing to achieve an appropriate purge decreases the operating rate.
Furthermore, purging of almost the entire device consumes a large amount of inert gas, increasing the running costs.
Japanese Patent Laid-Open No. 2001-028331 can achieve a primary object. That is, it can prevent or reduce impurities in a purge gas from intensively depositing on an optical element in a hermetically sealed casing. However, since the concentration of impurities in a purge gas and the flow rate of the purge gas are constant, the deposition of impurities cannot be reduced if impurities or a space containing impurities is exposed to light and the resulting photochemical reaction generates a deposit. Thus, it is difficult to reduce or prevent the fogging of an optical element.
Furthermore, even when the intensive deposition of impurities is prevented or reduced, deposition can occur at another portion. When the deposition of impurities occurs intensively at a certain portion, it is more effective to isolate this portion from the operating area of the optical system. However, in the structure according to the patent document, the deposition of impurities can occur on any optical element in a region to be purged and thereby decrease the transmittance of the contaminated optical element.
To avoid an increase in the size of an optical device, it is realistic to purge the optical device having a hermetically sealed structure and to prevent or reduce the fogging of an optical element in contact with the outside air by using an impurity filter installed in an environmental chamber. However, even when the impurity filter is installed in the environmental chamber, impurities may not be removed completely.
Furthermore, since the impurity filter can have a useful life depending on the amount of impurities to be removed, it needs laborious maintenance and additional running costs.